A family of unusual DNA structures has been discovered in segments with predominantly purines in one strand (pur.pyr sequences). These sequences are overrepresented in eukaryotic DNA and have been mapped near genes and recombination hot spots. When cloned into recombinant plasmids, many pur.pyr sequences are reactive to chemical and enzymic probes that are generally specific for single-stranded DNA. An intramolecular triplex is adopted by mirror repeats of G's and A's. Other non-B DNA structures adopted by similar sequences remain to be fully clarified but may be a family of related conformations. It is likely that these unorthodox structures play an important role in the function of the eukaryotic genome.

The extracellular matrix surrounding vertebrate oocytes is called the zona pellucida in mammals and perivitelline membrane (pvm) in birds. We have analyzed this structure in chicken follicles and laid eggs and have identified a 95-kDa component of the pvm, which, by protein sequencing, shows homology to mammalian zona pellucida proteins. Surprisingly, we could not detect this protein in ovarian granulosa cells or oocytes, but instead found high levels in the liver of the laying hen. In contrast, it is absent in rooster liver, but can be efficiently induced by estrogen treatment of the animal. An immunoscreen of a liver λ-ZAP library yielded a cDNA coding for a protein of 934 amino acids. It displayed significant homology to members of the ZP1/ZPB family from other species, notably to mouse and rat ZP1, and was therefore designated chkZP1. It is clearly different from a protein designated chkZPB that had been deposited in the database previously. Alignment of the known members of the ZP1/ZPB family demonstrated the existence of at least three subgroups, with representatives of both the ZP1 and the ZPB sequence homology group occurring in vertebrates. Northern blot analysis of liver extracts revealed the presence of a single 3.2 kb mRNA coding for chkZP1, distinct from the chkZPB transcript detectable in follicles. Immunohistochemical analysis of follicle sections demonstrates that chkZP1 can be found in the blood vessels of the theca cell layer as well as in the pvm surrounding the oocyte. Thus, in the chicken, at least one of the major pvm components is synthesized in the liver, and is transported via the bloodstream to the follicle.2 by guest on

The avian perivitelline membrane (PVM) is the site of initial contact between sperm and egg. It consists of only two major components, which are both homologues of the mammalian zona pellucida (ZP) proteins, and belong to the ZP1 and ZPC families, respectively. We have established a method to isolate large quantities of both native avian ZP proteins and have used these preparations to investigate their sperm-binding capacities. Chicken ZPC forms multimeric structures of defined size and binds to an approximately 180-kDa protein complex present in rooster sperm extracts. Based on experiments using both PVM and isolated proteins, we show that chicken ZP1 is proteolytically degraded by a sperm-associated protease but that chicken ZPC remains intact. An antiserum directed against chicken ZP1 is capable of inhibiting sperm binding to the PVM. Taken together, these data suggest that ZP1, in addition to ZPC, plays a major role in the initial interactions between sperm and egg.